1. Field of the Invention
A downhole acoustic logging tool is provided for imaging the texture and structure of the borehole sidewall. The signal level of the acoustic signals reflected from the sidewall are enhanced by minimizing the length of the travel-path of an acoustic-beam trajectory through highly attenuating borehole fluids.
2. Discussion of Related Art
Typical acoustic logging tools may include by way of example, a circumferential televiewer which comprises a rotating ultrasonic acoustic transducer that operates in a frequency range on the order of 100 kHz or more. Higher acoustic frequencies are preferred in order to achieve better resolution in the confined space of a borehole. In operation, the televiewer rotates at a desired rate such as 5 to 16 rotations per second to continuously scan the borehole sidewall as the televiewer is drawn up the borehole at a preferred rate of 3/16 to 3/8 inch per scan. A beam of acoustic pulses is launched along the normal to the borehole sidewall as the transducer scans the interior surface of the borehole. The pulse rate depends upon the desired spatial resolution such as 1500 pulses per second or 128 to 256 pulses per scan. The insonified borehole sidewall returns pulses reflected therefrom, back to the transducer on a time-multiplexed basis. The reflected acoustic signals are detected, amplified and displayed to provide a continuous picture of the texture and structure of the borehole sidewall. Other application include determination of the goodness of a cement bond to a steel casing as well as monitoring the integrity of the casing itself.
To protect the transducer element and the rotary mechanism from damage, the unit is ordinarily enclosed in an enclosure or cell that is sealed by an acoustic window from invasion by the ever-present borehole fluid. The acoustic window may be a relatively thin plastic boot which is pressure compensated.
The diameter of a borehole logger is on the order of 27/8 inches so that it can be run into relatively small boreholes. However many borehole diameters are on the order of ten to fourteen inches or more such that the length of the acoustic-pulse trajectory from the transducer, through the borehole fluid to the borehole sidewall, may be up to ten inches. In the normal course of events, the borehole fluid is contaminated by drill cuttings, air bubbles and foreign matter which severely attenuate the acoustic energy by scattering because the physical dimensions of the contaminants are comparable to the wavelength of the wavefields emitted by the transducer.
What is even more troublesome, however, is the complication that the acoustic attenuation coefficient in certain types of drilling fluid such as heavily-weighted oil-based muds is very high, on the order of 5 dB/cm (12.5 dB/inch). Remembering that the reflected acoustic signals must propagate over a two-way travel path, the maximum path length through the highly-attenuating drilling fluid must be kept well under 4 cm. Even that short path length may result in an attenuation of 20 dB.
Although it is true that the coefficient of attenuation diminishes with decreasing acoustic frequency, space considerations and resolution requirements do not permit the use of large, low-frequency transducers.
The term "attenuation coefficient" is sometimes referred to in the literature as the absorption constant, attenuation constant or simply attenuation. However, the term attenuation coefficient will be used to emphasize the fact that it is the signal strength that is important. The attenuation coefficient is not a constant but is a function of the frequency, characteristics and volume of the medium.
A number of petrophysicists have attempted to reduce the scattering and attenuation effects by reducing the trajectory path length of a pulsed acoustical beam by providing an enlarged, so-called mud excluder around the portion of the logging tool whereon is mounted the acoustic transducer. The mud excluder usually contains a volume of an acoustic transmission medium that minimizes the attenuation effect of the borehole fluids by replacing a volume of fluid characterized by a high attenuation coefficient with a medium that has a much lower attenuation coefficient.
M. L. Johnson in U.S. Pat. No. 3,390,737, issued Jul. 2, 1968 and assigned to Mobile Oil Corp., discloses a borehole liquid excluder which includes a thin-walled plastic boot adapted to surround the tool to displace undesirable borehole material from between the tool and the borehole wall. A coupling means releasably attaches the excluding means to the tool and allows the tool to be separated from the excluding means and be removed upwardly in the event that the excluding means becomes stuck in the borehole. The problem with this device is first, the acoustic window is rather flimsy; it can be easily damaged. The boot may actually be destroyed in a hot hole. Further, the acoustic window is parallel to the longitudinal axis of the logging tool rather than sloping and hence creates undesired ghost reflections by internal reflection.
Jorg Angher et al. in U.S. Pat. No. 4,711,122, issued Dec. 8, 1987 disclose a mud excluder that consists of a flexible casing that is attached to the borehole tool to form a cavity fitted around and adjacent to a rotating acoustic transducer. The cavity is filled with an acoustically low-loss fluid. The mud excluder includes a vertically planar acoustic window. The problem here is again the vertical window which causes internal reflection interference with received signals. Furthermore, the flexible casing is subject to rupture in the presence of a rough hole.
U.S. Pat. No. 4,382,290 issued May 3, 1983 to R. M. Havira provides a circumferential televiewer having a slanted acoustic exit window to reduce ghost reflections but does not teach or suggest that there is a problem with lengthy beam trajectories through the drilling fluid. The exit window is of plastic such as polyurethane.
A somewhat similar approach was taken by F. H. K. Rambow in U.S. Pat. No. 5,212,353, issued May 18, 1993. Whereas the '290 patent recommended a window slope of 20.degree. to 30.degree., the '353 patent suggests use of a 3.degree. to 5.degree. angle. Here again, there is no suggestion that the acoustic trajectory path length should be reduced. The '353 patent apparently depends entirely upon beam focussing using a curved transducer element which is said to substantially improve acoustic penetration through heavy mud. The acoustic window is formed of a polymethylpentane (TPX) plastic.
U.S. Pat. No. 4,876,672, issued Oct. 24, 1989 to S. C. Petermann et al. provides a borehole televiewer in which the rotating transducer is exposed directly to the borehole fluids during a logging operation. That arrangement is subject to damage to the exposed delicate transducer mechanism. There is no suggestion of trajectory path length reduction or abatement of ghost reflections.
There is a need for an acoustic logging tool that will maximize the signal-to-noise ratio of acoustic signals reflected from the sidewall of a borehole and one in which the internal ghost reflections are suppressed. The tool should be readily adaptable to different-sized boreholes, should be rugged and easily repairable after hard use and inexpensive to manufacture.